The present invention is directed toward an optical amplifier having a flattened gain curve.
Optical communication systems are a substantial and fast growing constituent of communication networks. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing (TDM) is frequently employed. In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a plurality of receivers, each detecting a respective channel by effectively filtering out the remaining channels.
Optical channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss "window" can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbiumn-doped fiber is "pumped" with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, as seen in FIG. 1, erbium doped fiber amplifiers do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm will be amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly-spaced optical channels. In these environments, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers. Thus, there is a need for an optical amplifier having flattened or uniform gain across its gain spectrum, i.e., an optical amplifier with a flattened gain curve.